1. Field of the Invention
The present invention relates to stacked dice packages and methods for fabricating the same. In particular, the present invention relates to standoffs of pillars positioned to separate microelectronic dice during fabrication of stacked dice packages.
2. State of the Art
Higher performance, reduced cost, increased miniaturization of integrated circuit components, and greater packaging densities of microelectronic devices are ongoing goals of the computer industry. One method of increasing the density of microelectronic device packages is to stack the individual microelectronic dice within the packages.
FIG. 19 illustrates an exemplary assembly 200 comprising a first microelectronic die 202 (such as a microprocessor, a chipset, a memory device, an ASIC, and the like) attached by a back surface 204 thereof to a substrate 212 (such as an interposer, a motherboard, a back surface of another microelectronic die, or the like) by a first layer of adhesive 214. A plurality of bond pads 216 is disposed on an active surface 206 of the first microelectronic die 202. The first microelectronic die bond pads 216 are generally placed near edges of the first microelectronic die active surface 206 and are electrically connected by a first plurality of bond wires 218 to corresponding first plurality of lands 222 on a surface 224 of the substrate 212.
A second microelectronic die 232 is attached by a back surface 234 thereof to the first microelectronic die active surface 206. A plurality of bond pads 242 is disposed on an active surface 236 of the second microelectronic die 232. The second microelectronic die bond pads 242 are generally placed near edges of the second microelectronic die active surface 236 and are electrically connected by a second plurality of bond wires 244 to a second plurality of lands 246 on the substrate surface 224. The first plurality of substrate lands 222 and the second plurality of substrates lands 246 are generally connected to conductive traces (not shown) that are in contact with external electrical connection devices, such as solder ball or pins (not shown), which connect the assembly 200 to external electrical devices (not shown).
When the second microelectronic die 232 is about the same size or larger than the first microelectronic die 202, such that the first plurality of wire bonds 218 may be contacted by the second microelectronic die 232, a standoff is necessary to raise the second microelectronic die 232 above the first microelectronic die 202 to give clearance for the first plurality of wire bonds 218. As shown in FIG. 17, a thick layer of die attach adhesive 252 (such as epoxies, urethane, polyurethane, silicone elastomers, and the like) may be disposed between the first microelectronic die active surface 206 and the second microelectronic die back surface 234. Thereafter, an encapsulation material 254 is disposed to cover the first microelectronic die 202 and the second microelectronic die 232.
One problem that must be addressed in the connection of various different types of materials (i.e., microelectronic dices, adhesives, encapsulation materials, etc.) is the coefficient of thermal expansion (xe2x80x9cCTExe2x80x9d) for each material. The CTE is a measurement of the expansion and contraction of each material during heating and cooling cycles, respectively. These heating and cooling cycles occur during the operation of the microelectronic device 202 and during power up and power down of the microelectronic device 202.
The use of a thick die attach adhesive layer 252 can cause stresses due to a mismatch between the CTE of the thick die attach adhesive layer 252 and the microelectronic dice (first microelectronic die 202 and second microelectronic die 232) as the assembly 200 heats to a normal operating temperature when on and room temperature when off. Furthermore, materials used for the thick die attach adhesive layer 252 generally shrink during the curing process, which also places stresses on the first microelectronic die 202 and the second microelectronic die 232. Stresses due to CTE mismatch and curing increase the probability that cracks will initiate and propagate in both the first microelectronic die 202 and the second microelectronic die 232. These cracks may cause the failure of the first microelectronic die 202 and/or the second microelectronic die 232.
Another problem with thick die attach adhesive layers 252 is their tendency to absorb moisture, which can have adverse affects on the first microelectronic die 202 and the second microelectronic die 232. A further problem is proper flow control of the adhesive material. Improperly applied adhesive can interfere with the first plurality of bond wires 218 and first plurality of bond pads 216.
As shown in FIG. 20, to overcome for the problems with the use of thick die attach adhesive layers 252 (see FIG. 19), a spacer 266 (such as silicon, having about the same CTE as the first microelectronic die 202 and the second microelectronic die 232) may be attached to the first microelectronic die active surface 206 with a first, thin die spacer adhesive layer 262 and attached to the second microelectronic die back surface 234 with a second, thin die spacer adhesive layer 264 to form a package 270. However, using the spacer 262 involves additional processing steps and presents alignment problems, which increases the cost of the package 270.
Furthermore, with both packages described above, the CTE can cause delamination between the adhesive layers and the microelectronic dice.
Therefore, it would be advantageous to develop a stacked package providing adequate spacing between the stacked microelectronic dice that does not have the disadvantages of thick adhesive layers or of spacers.